Design Expressions and Dynamic Evaluation of CFST Bridges Subjected to Seismic Hazards

Thesis (Ph.D.)--University of Washington, 2016-06

Concrete filled steel tubes (CFSTs) consist of a steel tube with concrete infill. CFST columns offer an effcient and economical alternative to conventional reinforced concrete bridge columns, in that CFST columns can offer larger strength and stiffness and facilitate rapid construction. The first phase of this research project uses numerical and experimental methods to develop design procedures for various CFST column-to-cap beam connections. Three connections were studied, namely: (1) an embedded ange connection in which the CFST is embedded in the cap beam, (2) a welded dowel connection in which a series of headed dowels is welded inside the tube and embedded into the cap beam (both fully bonded and partially deboned conditions were studied), and (3) a dowel connection in which an independent cage of headed dowels and transverse reinforcing is developed into the CFST and embedded into the cap beam. All specimens were constructed using precast cap-beam components to demonstrate the feasibility of using these connections for accelerated bridge construction (ABC). The experimental program included specimens that simulated either the transverse or longitudinal direction of the bridge. Results showed that the three connection types provide excellent ductility under reversed-cyclic loading while all super-structure elements remained essentially elastic. The document provides design expressions, design examples and proposed codified language to facilitate immediate implementation of the research results into practice. In the second phase, performance based earthquake engineering (PBEE) tools were used to evaluate and compare the response of a reinforced concrete (RC) bridge with a CFST bridge designed using the design expressions developed in phase 1. The PBEE tool set included fragility functions that express the likelihood of damage and reparability, nonlinear analysis methods that capture the full cyclic response of the structural system, and incremental dynamic analyses (IDAs) to evaluate collapse potential. The analysis process was as follows: (1) the selected bridge was designed using RC and CFST components, with the objective of having the same strength and stiffness, (2) the structures were subjected to a suite of ground motions scaled to represent servicability (10% probability of exceedance in 50 years), design (7% probability of exceedance in 75 years), and maximum considered (2% probability of exceedance in 50 years) seismic hazard levels, (3) fragility curves were used to compare the performance of the bridge including damage state as well as maximum and residual drift, and (4) IDAs were conducted to compare the collapse probability. Results indicate that the CFST structure is more resilient than the RC structure by all measures.